The present invention relaters to a copper-foiled laminated sheet for flexible printed circuit board as well as a flexible printed circuit board or sheet prepared thereof.
Along with the rapid development of electronic information-processing and communication instruments in recent years, many of the electric or electronic circuits used therein are formed in the form of a printed circuit board, which is prepared usually by the photolithographic techniques or screen printing followed by etching using a copper-foiled laminate as the base material. Such a copper-foiled laminate is a composite plate or sheet formed of an electrically insulating plate or sheet of a plastic resin such as polyethylene terephthalates and polyimides as the base to which a copper foil is adhesively bonded in lamination. In compliance with the demand for more and more compact and lighter and lighter instruments, many of the printed circuit boards or sheets are prepared using a thin and flexible laminated sheet as the material for the patterning process by partly removing the copper foil thereon by etching so that the remaining copper foil constitutes the desired electric circuit.
It is also highly desirable that the printed circuit board or sheet is see-through to such an extent that the assembly of the parts of an instrument or a display below the circuit sheet is visible as completely as possible through the printed circuit sheet above it. This requirement can of course be satisfied by using a plastic-made base sheet or film as highly transparent as possible and by removing the copper foil as widely as possible to leave only a small area of the copper foil.
Commercially available copper foils include those manufactured by an electrolytic process and those manufactured by rolling. Elec-rolytic copper foils usually have a relatively rough surface with good receptivity of an adhesive so that they can be used as such for the adhesive bonding with the insulating base film of plastics. Rolled copper foils, on the other hand, usually have a smooth surface with poor receptivity of an adhesive so that they cannot be used practically for the manufacture of copper-foiled laminated sheets despite their relative inexpensiveness unless the surface thereof is subjected to a roughening treatment prior to adhesive bonding with a plastic film. At any rate, it is important that the surface of the copper foil to be adhesively bonded with the plastic film should have an adequate surface roughness of, for example, at least 10 according to the standard specified in JIS B 0601.
The use of such a surface-roughened copper foil in the preparation of a copper-foiled laminated sheet has a problem in respect of the see-throughness of the flexible printed circuit sheet manufactured thereof. Namely, when a part of the copper foil on the laminated sheet has been removed by the process of etching to construct an electric circuit, the surface of the layer of the adhesive which formerly is in service for bonding of the plastic film and the copper foil is now exposed bare while the surface of the adhesive layer is not smooth enough but in a condition with ruggedness which is a replica of the rough surface of the copper foil having been removed by etching. Needless to say, such a surface condition with roughness is very detrimental when high see-throughness or transparency is desired of the sheet or film because the light transmitting the sheet or film like a frosted window pane is irregularly diffused at the roughened surface so that the bodies below the film or sheet are hardly visible therethrough. The see-througness of a film or sheet is determined by the % transmission of parallel light beams as defined in JIS K 6714. In other words, the parallel light-beam transmission is a determinant factor of see-throughness along with the overall light transmission. In this connection, it is not rare that the parallel light-beam transmission through the areas of a flexible printed circuit sheet from which the copper foil has been removed by etching is 20% or less. It is of course that such a film or sheet having the parallel light-beam transmission so low can hardly satisfy the requirement that the flexible printed circuit sheet should be highly see-through.